I'm writing this during a cold spell, so let's create a giant frost mammoth.
However, a frost mammoth shouldn't just be a big, hairy animal. Oh, no. A frost mammoth is an avatar of winter, a force of nature that can use the ice itself as a weapon. Any normal person who stumbles across a frost mammoth wouldn't dream of disturbing it and risk its wrath. Its eyes are said to bewitch and doom those who dare approach it.
To build it, I'm going to take the CR 10 Guardian Naga as a base, pull in some elements from the CR 6 Mammoth, and replace the Naga's spells with several unique magical abilities.
The frost mammoth shoudl have a special fearful presence. Instead of just causing enemies to run, it'll cause them to take one of four possible actions. This will conveniently let the DM either decide on the most effective action or let a d4 decide. Enemies anywhere near a frost mammoth who can also see it must make a Charisma save each turn or do one of the following: lose their movement, move away from the mammoth, roll one Hit Die and lose that much HP, or fail all attacks this round.
As an intensely magical creature, it will also affect a variety of magical spells, requiring spellcasters to re-thinking their strategies. Just as intense cold clears the head and reminds you of the life pulsing through your body, spells cast from the illusion and necromancy schools fail near a frost mammoth, while abjuration and transmutation spells are cast at +2.
I'll add to this a fearful gaze that works a bit like a Beholder's eye ray. If a mammoth looks at you, you must make a save or feel an immense blast of cold chilling you to your core.
Cool! (Sorry.) Let's stat it up.
Homebrewery stat block:
Huge beast, chaotic neutral
- Armor Class 18
- Hit Points 150 (15d12 + 52)
- Speed 40 ft.
STR DEX CON INT WIS CHA 24 (+7) 18 (+4) 16 (+3) 16 (+3) 19 (+4) 18 (+4)
- Proficiency Bonus +4
- Saving Throws Int +7, Cha +8, Con +7, Dex +8, Wis +8
- Damage Immunities cold
- Condition Immunities frightened
- Senses darkvision 60 ft., passive Perception 14
- Challenge 10 (5900 XP)
Ripples in the Weave. All spells from the illusion and necromancy schools cast within 160 feet of the frost mammoth automatically fail. All spells from the abjuration and transformation schools cast within 160 feet of the frost mammoth gain an automatic +2 on the spell roll.
Awesome Presence. Any creature that starts its turn within 40 feet of the frost mammoth must make a DC 15 Charisma check at the beginning of its turn or do oe of the following (DM's choice or roll a die):
- Lose its movement
- Use all its movement moving away from the frost mammoth
- Roll one Hit Die and lose that much HP (the HD is not consumed)
- Fail all attacks this round
Gaze of the Ancients. Ranged Weapon Attack: +10 to hit, range 40/80 ft., one creature. Hit: The target must make a DC 15 Constitution saving throw, taking 45 (10d8) frost damage on a failed save, or half as much damage on a successful one.
Stomp. Melee Weapon Attack: +10 to hit, reach 5 ft., one creature. Hit: 29 (4d10 + 7) bludgeoning damage.